1. Field of the Invention
The present invention is related to the structure of a ball bump of wire bonding and the formation thereof and more particularly to the formation of the ball bump with a bonding portion that provides a flat upper surface for wire bonding.
2. Description of the Related Art
To avoid the destruction of the bonding pads during wire bonding onto the bonding pad of the semiconductor device, the ball bump is arranged on the bonding pad in such a way as to protect it. Although the ball bump protects the bonding pad directly from being struck, wire bonding on an irregular upper surface of the ball bump increases the variability of results. Providing that a bonding machine uses the same set of wire bonding parameters, an irregular upper surface reduces the quality and reliability of wire bonding of the semiconductor device.
The prior art of the U.S. Pat. No. 5,858,149, issued on Jan. 12, 1999 to Seo et al., discloses the structure of a ball bump and the formation thereof As shown in FIG. 1, the chip 100 has a bonding pad 101. A bonding tool 110, located above the chip 100, holds a wire 120 which is melted to form a ball 121. As shown in FIG. 2, the bonding tool 110 strike the ball onto the bonding pad 101 of the chip 100 such that the bottom of the ball adheres to the bonding pad 101. Then, the clamp 111 of the bonding tool 110 is opened and the bonding tool 110 presses the ball bump 130. As shown in FIG. 3, the bonding tool 110 is moved upward a predetermined distance so as to reserve an appropriate distance of wire 120. The clamp 111 is then closed to hold the wire 120, and the bonding tool 110 is moved upward to pull the wire 120. The ball bump 130 and the wire 120 are cut along the heat-affected zone and leave the ball bump 130 on the bonding pad 101. However, the upper of the ball bump 130 forms a tip 131 whose height is determined by the heat affected-zone of the wire 120; so tip 131 varies in accordance with the heat-affected zone and increases the variability of wire bonding. When the bonding tool 110 directly pushes the tip 131 in a horizontal direction, this results in the collapse and shift of tip 131, and the irregular upper surface of the tip 131 increases the variability of the ball bump 130.
The present invention intends to provide a structure for the ball bump which includes a bonding portion on which a flat upper surface of uniform height is formed so as to reduce the variability of wire bonding in such a way as to mitigate and overcome the above problem.